This invention is concerned with surface structures having improved structural characteristics over conventional shaped panels, such as may be used in the manufacture of vehicles or in other applications, such as aerospace, industrial applications, military applications, and recreational products. The invention is directed at replacing currently used materials in existing applications, as well as providing new materials for use in new or existing applications.
Panels are used in a wide variety of applications, and come in a wide variety of materials and shapes. Some panels are flat, and some are bent or stamped into a shape. For example, a body panel on an automobile might be stamped from sheet steel or other alloy. In some cases, such a panel might be too weak to act as a structural component of the assembly, and is fastened to a frame or chassis. In other instances, the panel may be sufficiently strong to provide structural support, or may be welded or otherwise joined with other sheet steel parts to form an assembly that includes the panel portion for use in the final product.
Unfortunately, a stronger panel generally means a thicker panel. It takes more energy to form a panel into a given shape if the material is thicker, and thicker material might limit the shapes that can be formed. Also, a thicker panel is heavier. Thus, simply making a thicker panel to obtain strength adds to both the material costs and the fabrication costs, as well as the weight of the final product, particularly if a panel is made from steel sheet.
Thinner panel pieces are often shaped and then spot-welded into a box-like assembly; The various pieces support each other to make a structural element of desired strength and rigidity, which often weighs less than either a thin sheet attached to a frame, or a thick structural panel. However, thinner panels are not without problems, such as denting and rust-through. Thin sheet steel dents relatively easily, even from minor impacts, and might rust entirely through in a short time if corrosion protection fails.
Alternatives to making body panels from stamped steel sheet have been developed to overcome some of the limitations of steel panels. One approach has been to fabricate panels, or even complete bodies, out of re-enforced resin (so-called xe2x80x9cfiber-glassxe2x80x9d) composites. Fiber-glass parts are generally lighter than comparative steel parts, allow greater choice in the types of shapes that may be fabricated, and do not rust. Fiber-glass parts are typically attached to a frame, as they are typically not structural, as they tend to fail under strain when in sheet form, although sometimes a shape is built up or adhesively attached to provide mechanical strength. However, fiber-glass parts tend to crack or splinter on impact, rather than absorbing much energy from the impact, and are considered to be relatively fragile, and scratch easily. Other re-enforced resin systems, such as xe2x80x9ccarbon-fiberxe2x80x9d systems have been developed to improve some of the shortcomings of fiber-glass parts, but are typically more difficult to work with, and more expensive. In some applications, fiber-glass or other composite parts are molded into a thick, structural part of complex (multiple curved surfaces) part.
Molded parts and stamped steel or other alloy parts both are typically made in a process using a two-part mold or two-part stamping die, respectively. Making two-part mold tooling is fairly straight forward but the resulting sprues and seams must be trimmed from the part before the part can be considered finished. Also, the material, which is typically injected into the mold in a fluid form, must typically be left in the mold long enough to solidify, either by cooling or by chemical reaction, to solidify enough to retain the shape of the mold upon removal. The mold dwell time can slow down the entire fabrication process, thus increasing costs.
Stamping steel or other alloys also requires substantial tooling costs. A stamp and die are both precision tools that must match, and that typically accept a particular material of a particular thickness. Changing the design of a stamped part is expensive and time consuming. Stamping steel sheet or other alloy has other problems that limit the type of shapes that can be formed. For example, there is a certain maximum depth, also known as aspect ratio, that a particular sheet can be drawn to. Trying to stamp sheet into certain shapes can cause pulling and stretching of the sheet, particularly puckering, or webbing, in areas adjoining the seam of a curved area.
Therefore, a panel that may take a shape with complex curves that is light, strong, dent resistant, and corrosion resistant is desirable, and a method for making such panels that is efficient and adaptable to various materials and shapes is further desirable.
A surface structure including a skin portion bonded to a build-up section and a backing provides a light, strong, versatile structural element. In one embodiment, the build-up section includes cells, the cell walls being essentially perpendicular to a curved surface of the skin portion. In one instance, gas pressure is used to deform an aluminum sheet metal skin preform to a desired shape and thermoplastic build-up material is applied in a desired pattern to the formed skin. The desired pattern is initially hexagonal, forming a xe2x80x9choneycombxe2x80x9d pattern of cells adjacent to the skin, then the cell walls transition to a circular, columnar section. A backing of aluminum sheet is then formed to the build-up, again using gas pressure.
These and other aspects of the invention will be better understood by reference to the following detailed description in connection with the accompanying drawings.